1. Field of the Invention
This invention generally relates to flexible substrate based ball grid array (BGA) packages, and more particularly to flexible film substrates for use in forming flexible substrate based BGA packages and manufacturing methods thereof.
2. Description of the Related Art
FIG. 1 depicts a conventional flexible substrate based BGA package 100 typically utilizing a flexible film substrate 110 (see FIG. 2) for carrying a semiconductor chip 120. The flexible film substrate 110 is provided with a plurality of chip connection pads 110a arranged about the periphery of the semiconductor chip 120. The semiconductor chip 120 is securely attached onto the flexible film substrate 110 through a nonconductive adhesive (e.g., epoxy resin) 122 and electrically connected to the chip connection pads 110a through a plurality of bonding wires 130. The chip connection pads 110a are electrically connected to a plurality of solder pads 110b through conductive traces 110d. The flexible film substrate 110 has a plurality of through-hole 110c disposed corresponding to solder pads 110b. Each solder pad 110b has a portion exposed within the corresponding through-hole 110c for mounting a solder ball 140. The upper surface of the flexible film substrate 110, the semiconductor chip 120 and the bonding wires 130 are encapsulated in a package body 150. The package body 150 is formed from insulating material such as epoxy. The flexible substrate based BGA package 100 is mounted to a substrate (not shown), such as a printed circuit board, through the solder balls 140.
Typically, the surfaces of the chip connection pads 110a and solder pads 110b are plated with a layer of metal (not shown) such as gold which bonds well with conventional bonding wire material. However, gold does not bond well with the nonconductive adhesive 122, so the central part of the chip attaching area is not provided with solder pads in order to enhance the adhesion between the flexible film substrate 110 and the nonconductive adhesive 122, whereby the semiconductor chip 120 is more securely attached onto the flexible film substrate 110. However, since the solder pads 110b are not evenly distributed on the chip attaching area 160 of the flexible film substrate 110, the central part thereof is prone to be deformed by the stress due to CTE (coefficient of thermal expansion) mismatch between the central part and other part of the chip attaching area 160. This may create problems of delamination between the chip and the substrate or die cracking.
It is a primary object of the present invention to provide a flexible substrate based BGA package comprising a flexible film substrate having a die attaching area for carrying a semiconductor chip, wherein the flexible film substrate is provided with at least a dummy pad disposed centrally on the chip attaching area, thereby reducing problems of delamination between the chip and the substrate or die cracking.
It is another object of the present invention to provide a flexible substrate based BGA package comprising a semiconductor chip securely attached onto a die attaching area of a flexible film substrate through a nonconductive adhesive, wherein the flexible film substrate is provided with at least a dummy pad disposed centrally on the chip attaching area, and the dummy pad has a cupric oxide coating formed thereon for enhancing its adhesion with the nonconductive adhesive.
A flexible substrate based BGA package in accordance with a preferred embodiment of the present invention generally comprises a semiconductor chip securely attached onto a flexible film substrate through a nonconductive adhesive. The flexible film substrate includes a flexible film having a chip attaching area for carrying the semiconductor chip. The upper surface of the flexible film is provided with a plurality of chip connection pads, a plurality of solder pads, and at least a dummy pad which is disposed centrally on the chip attaching area. Preferably, the surface of the dummy pad has a cupric oxide coating for enhancing its adhesion with the nonconductive adhesive. The chip connection pads are arranged about the periphery of the chip attaching area. The solder pads are disposed about the dummy pad(s) and electrically connected to the corresponding chip connection pads. The flexible film has a plurality of through-holes formed corresponding to the solder pads such that each solder pad has a portion exposed within the corresponding through-hole for mounting a solder ball. A package body is formed over the semiconductor chip and the upper surface of the flexible film substrate.
According to the flexible substrate based BGA package of the present invention, since at least a dummy pad is disposed centrally on the chip attaching area of the flexible film substrate, the central part of the chip attaching area of the substrate has a better rigidity and strength to resist external forces thereby reducing the problems of die cracking or delamination. Further, the cupric oxide coating on the surface of the dummy pad has a contour of roughness, so the bonding mechanism of the interface between the cupric oxide coating and the nonconductive adhesive includes chemical bonding as well as mechanical interlock thereby greatly enhancing the adhesion between the dummy pad and the nonconductive adhesive thereby reducing the occurrence of delamination.
The present invention further provides a method for producing a flexible film substrate comprising the steps of: (A) providing a flexible film having opposing upper and lower surfaces, the upper surface of the flexible film has a chip attaching area adapted for carrying a semiconductor chip; (B) forming a plurality of through-holes in the flexible film; (C) laminating a metal layer on the upper surface of the flexible film; and (D) etching the metal layer to form a plurality of solder pads, chip connection pads, conductive traces, and at least a dummy pad, wherein the solder pads are disposed corresponding to the through-holes and electrically connected to the chip connection pads through the conductive traces, and the dummy pad is disposed centrally on the chip attaching area.